http://www.swpc.noaa.gov/rt_plots/kp_3d.html
http://www.swpc.noaa.gov/rt_plots/Kp.gif
spaceweather.com
SUBSIDING STORM: A polar geomagnetic storm that began during the late hours of April 11th is subsiding now. At its peak, the storm registered 5 on the Kp scale of magnetic disturbances, which means it was a relatively minor event. Nevertheless, it was strong enough to spark naked-eye auroras. A family of "aurora addicts" traveling through Boden, Sweden, took this picture of the light: